In sheet-fed offset printing machines, a printing plate is fastened onto the plate cylinder by means of a clamping rail adjacent to the leading edge (LE) of the plate cylinder and another clamping rail adjacent to the trailing edge (TE). Typically, these clamping rails are arranged in a pit on the plate cylinder. To change a printing plate, the clamping rail at the trailing edge is first opened and the printing plate is progressively separated from the plate cylinder by rotating the latter backwards, until only the front edge portion of the plate remains in contact with the cylinder. Then, the clamping rail at the leading edge is opened and the plate is removed. Next, a new printing plate is laid with its front edge in the leading edge clamping rail. After proper alignment, the front edge of the new plate is clamped by closing the leading edge clamping rail, and the plate cylinder is then rotated in the forward direction until the rear edge of the printing plate can be fed into the trailing edge clamping rail and clamped there. The tensioning of the printing plate is carried out via the trailing edge clamping rail.
In conventional printing machines, these steps are carried out manually by the operator and are correspondingly time-consuming. In order to shorten the setting-up time, automatic or semi-automatic printing plate change systems have recently become known, in which the used printing plate is taken from the plate cylinder into a magazine or a storage chamber and a new printing plate is taken from this same magazine or storage chamber and fed to the plate cylinder. For this purpose, the plate cylinder has devices with which the clamping and tensioning processes can be initiated by remote control.
German Patent DE 4 130 359 A2 discloses a device and a method for changing printing plates of a printing machine, in which the printing plate change can be carried out both semi-automatically and fully automatically. In the semi-automatic mode of operation, it is provided that the operator controls the change process by actuation of multiple operating elements. Due to the number of partial processes to be initiated in order to carry out the plate change, this method and the corresponding device can be quite costly.
Disclosed in U.S. Pat. No. 5,111,744 (and corresponding German Patent DE 3 940 796 C2) is a method and a device for the automatic changing of a printing plate, in which a so-called position detector determines the in-register laying of the printing plate in the leading edge rail, whereupon clamping is initiated.
Determining the in-register position by means of register pins insulated electrically with respect to the plate cylinder and which interact with corresponding stamped-out regions in a printing plate, is disclosed in European Patent EP 0 555 782 A1. If the printing plate is lying with its stamped-out regions positioned correctly on these register pins, a current loop is closed. This is detected and used to determine when the plate is properly positioned.
A printing plate draw-in system operating semiautomatically is known under the designation PPL (Power Plate Loading) from the firm MAN Roland Druckmaschinen AG and is in use on machines of the series R 700. This device is demonstrated in the German magazine Druckwelt (Printing World) 4/25.02.1993, page 24 ff, and is further described in German Patent Application DE 4 215 969 A1 and the corresponding U.S. Pat. No. 5,331,892. In this device, the operator only removes a printing plate, separated from the plate cylinder, through a slit in the sliding guard and feeds a new printing plate to the plate cylinder. For this purpose, the system is provided with a pneumatically movable sliding guard having a guide device which can be pivoted in and out relative to the plate cylinder. The remotely actuated clamping and unclamping and the corresponding positioning processes of the plate cylinder are initiated in this system by the operator via operating elements.